FeNC Catalysts Research Articles

Heteroatom-Doped Carbon Nanostructures as Oxygen Reduction Reaction Catalysts in Acidic Media: An Overview

This paper provides an overview of the studies that were conducted in our laboratories in the last decade on hetero-atom doped carbon structures as potential oxygen reduction reaction (ORR) catalysts for PEM fuel cells. These studies include evaluating the potential of nitrogen doped carbon nanostructures as cathode catalysts for proton exchange membrane and direct methanol fuel cells and examining the nature of the transition metal used as growth catalysts in synthesis of these materials, through activity and in situ and ex situ characterization experiments. These studies also shed some light on the ongoing debate about the differences and similarities between two classes of materials used for ORR, namely FeNC and CNx catalysts, through activity and stability tests, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, Mossbauer spectroscopy, temperature-programmed techniques and selective poisoning experiments designed to probe the active sites.

Use of H2S to Probe the Active Sites in FeNC Catalysts for the Oxygen Reduction Reaction (ORR) in Acidic Media

H2S has been used as a probe molecule both in an “in situ” poisoning experiment and in intermediate-temperature heat-treatment steps during and after the preparation of FeNC catalysts in an attempt to analyze its effect on their ORR activity. The heat treatments were employed either on the ball-milled precursor of FeNC or after the Ar-NH3 high temperature heat treatments. ORR activity of the H2S-treated catalysts was seen to be significantly lower than the sulfur-free catalysts, whether the sulfur exposure was during a half-cell testing, or as an intermediate-temperature exposure to H2S. The incorporation of sulfur species and interaction of Fe with sulfur were confirmed by characterization using XPS, EXAFS, TPO, and TPD. This study provides crucial evidence regarding differences in active sites in FeNC versus nitrogen-containing carbon nanostructured (CNx) catalysts.

Fe[sbnd]N[sbnd]C catalyst modified graphene sponge as a cathode material for lithium-oxygen battery

The cathode of a lithium-oxygen battery needs the synergism of a porous conducting material and a catalyst to facilitate the formation and decomposition of lithium peroxide. Here we introduce a graphene sponge (GS) modified with FeNC catalyst for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). The porous, 3-dimensional conductive and free standing nature of the graphene sponge makes it become excellent skeleton of cathode for lithium-oxygen battery. The FeNC catalyst nanoparticles dispersed uniformly on the graphene sheets show excellent catalytic reactivity in both discharge and charge processes. This kind of composite material greatly improves the capacity and cyclability of the lithium-oxygen battery. With dimethyl sulphoxide as electrolyte, the capacity reaches 6762mAhg−1 which is twice of the pure graphene sponge. In addition, the cell containing FeNGS air electrode exhibits stable cyclic performance and effective reduction of charge potential plateau, indicating that FeNGS is promising as an OER catalyst in rechargeable lithium-air batteries.

Impact of transition metal on nitrogen retention and activity of iron–nitrogen–carbon oxygen reduction catalysts

Iron based nitrogen doped carbon (FeNC) catalysts are synthesized by high-pressure pyrolysis of carbon and melamine with varying amounts of iron acetate in a closed, constant-volume reactor. The optimum nominal amount of Fe (1.2 wt%) in FeNC catalysts is established through oxygen reduction reaction (ORR) polarization. Since the quantity of iron used in FeNCs is very small, the amount of Fe retained in FeNC catalysts after leaching is determined by UV-VIS spectroscopy. As nitrogen is considered to be a component of active sites, the amount of bulk and surface nitrogen retention in FeNC catalysts are measured using elemental analysis and X-ray photoelectron spectroscopy, respectively. It is found that increasing nominal Fe content in FeNC catalysts leads to a decreased level of nitrogen retention. Thermogravimetric analysis demonstrates that increasing nominal Fe content leads to increased weight loss during pyrolysis, particularly at high temperatures. Catalysts are also prepared in the absence of iron source, and with iron removed by washing with hot aqua regia post-pyrolysis. FeNC catalysts prepared with no Fe show high retained nitrogen content but poor ORR activity, and aqua regia washed catalysts demonstrate similar activity to Fe-free catalysts, indicating that Fe is an active site component.

Oxygen reduction activities compared in rotating-disk electrode and proton exchange membrane fuel cells for highly active Fe[sbnd]N[sbnd]C catalysts

In the past three years, two novel synthesis methods for non-precious metal catalysts resulting in a breakthrough of their activity and performance at the cathode of the proton-exchange membrane fuel cell (PEMFC) have been reported by the group of Prof. Dodelet. While the activity of these novel Fe-based catalysts for the oxygen reduction reaction is very high in PEMFC, our preliminary activity measurements with the rotating disk electrode (RDE) technique on one of them showed an activity being a factor 30–100 lower than the one measured in PEMFC at 80°C.The present work explains to a large extent this huge difference. Two FeNC catalysts synthesized via our novel approaches and one FeNC catalyst synthesized via our classical approach were investigated in RDE and PEMFC. In both systems, the effect of the ink formulation (Nafion-to-catalyst ratio) was investigated. Optimization of the RDE ink formulation explains a factor between 5 and 10 in the two-decade gap mentioned above. Then, the effect of temperature in the RDE system was investigated. An increase from 20 to 80°C was found to result in a theoretical maximum twofold increase in activity. However, in practice, decreased O2 solubility with increased temperature cancels this effect.After taking into account these two parameters, a difference in ORR activity between RDE and PEMFC of ca a factor five still remained for one of the two novel FeNC catalysts investigated here. The lower initial activity measured in RDE for this catalyst is shown to be due to the fast adsorption of anions (HSO4−) from the liquid H2SO4 electrolyte on protonated nitrogen atoms (NH+) found on its surface. The phenomenon of anion adsorption and associated decreased ORR activity also applies to the other novel FeNC catalyst, but is slower and does not immediately occur in RDE.